1. Field of the Invention
The present invention relates to an image forming method and apparatus, which form images using a recording technology such as electrophotography and electrostatography. In addition, the present invention relates to toner for use in image forming apparatus.
2. Discussion of the Background
Electrophotographic image forming apparatus typically perform the following image forming processes.    (1) Charging a surface (i.e., an image forming area) of an image bearing member such as photoreceptors (charging process):    (2) Irradiating the charged image bearing member with light to form an electrostatic latent image on the image bearing member (light irradiation process);    (3) Developing the electrostatic latent image with a developer including a toner to form a toner image on the image bearing member (development process);    (4) Transferring the toner image onto a receiving material fed from a sheet feeding device optionally via an intermediate transfer medium (transfer process);    (5) Fixing the toner image to the receiving material upon application of heat and pressure thereto (fixing process); and    (6) Removing toner particles remaining on the image bearing member and intermediate transfer medium without being transferred so that the image bearing member and intermediate transfer medium are ready for the next image forming processes (cleaning process).
Charging methods for charging an image bearing member are broadly classified into DC charging methods in which a DC voltage is applied to an image bearing member, and AC charging methods in which a DC voltage on which an AC voltage is superimposed is applied to an image bearing member.
Since AC charging methods have advantages such that the image bearing members can be uniformly charged and the potentials of the charged image bearing members can be easily controlled, the AC charging methods are broadly used for low-speed or medium-speed electrophotographic image forming apparatus. Charging rollers are typically used as the charging member. The charging methods using a charging roller are hereinafter referred to as AC roller charging methods. Among the AC roller charging methods, short-range AC roller charging methods, in which a charging roller arranged in the vicinity of an image bearing member with a small gap therebetween is used for charging an image bearing member, are broadly used particularly for durable-use image forming apparatus attaching importance to durability. This is because the degree of contamination of the charging rollers used therefor is smaller than that of charging rollers used for contact roller charging methods, resulting in improvement of durability of the charging rollers.
In AC roller charging methods, at least a certain number of electric discharges have to be performed per unit length in the sub-scanning direction in order to uniformly charge the image bearing member. In general, the frequency of the AC voltage is set to be from 6.0 to 7.3 cycles per millimeter.
Since AC charging methods repeat negative and positive electric discharges, the AC charging methods expose the image bearing members to relatively serious hazards compared to DC roller charging methods and corona charging methods. Specifically, AC charging methods tend to easily cause problems in that the surface of the image bearing members (such as photoreceptors) is abraded, and/or foreign materials such as toner particles and dust of paper sheets used as a receiving material are adhered to the surface of the image bearing members, resulting in formation of a film (such as toner film) on the surface of the image bearing members (hereinafter referred to as a filming problem), thereby deteriorating image qualities. This is a drawback of the AC charging methods. These problems are typically caused when the image forming apparatus are high speed image forming apparatus, or large-size image forming apparatus capable of forming images with an A3 size or a size slightly greater than A3 size. The reason therefor is that in these image forming apparatus, the diameter of the charging roller is larger (i.e., the curvature of the charging roller is smaller) than that in low speed image forming apparatus, and therefore the discharge area of the charging roller increases, resulting in increase of the risk of AC charging problems. In order to further improve the image qualities and to uniformly charging image bearing members, the AC frequency is preferably from 7.5 to 8.0 cycle/mm.
For the reason mentioned above, high speed image forming apparatus with a linear speed of 250 mm/s or more hardly use an AC roller charging method. However, there are some high speed image forming apparatus using an AC roller charging method. Such image forming apparatus use a short-range charging method, and therefore the charging roller has good resistance to contamination, i.e., the charging roller has a relatively long life. However, the gap between the charging roller and the image bearing member increases in such image forming apparatus, thereby increasing the risk of the filming problem. In attempting to prevent occurrence of the filming problem, a lubricant such as zinc stearate is coated on the surface of the image bearing member. Therefore, a large-size lubricant has to be set in the image forming apparatus to lengthen the life of the image bearing member, resulting in jumboization of the image forming apparatus.
Toners prepared by a pulverization method including kneading toner components such as binder resins, colorants (pigments), charge controlling agents, and release agents while heating; cooling the kneaded mixture; pulverizing the kneaded mixture; and then classifying the pulverized mixture, have been typically used for image forming apparatus. However, it is difficult for such a pulverization method to control the particle diameter and shapes of the toner particles. In attempting to control the particle diameter and shapes of toner particles, polymerization methods (such as emulsion polymerization methods, and solution dispersing methods) in which toner particles are prepared in an aqueous medium have been actively developed and used.
In addition, a particulate material having an average particle diameter of from 5 to 25 nm is added as an external additive to toner particles, followed by mixing to improve the fluidity and chargeability of the toner. For example, techniques such that silica or titanium dioxide is added as an external additive to toner to improve the fluidity of the toner have been proposed. In addition, techniques such that an external additive whose surface is treated with an organic silicon compound such as silane and silicone oils to improve the hydrophobicity and chargeability of the toner have been proposed. Further, techniques such that a titanium oxide having a low electric resistance is used as an external additive have been proposed.
However, when the toner having such an external additive thereon is mixed with a carrier to be used for a two-component developer, a problem in that the external additive is embedded into toner particles when receiving stress from the carrier, resulting in deterioration of the fluidity of the toner tends to occur. In attempting to prevent occurrence of the problem, it is effective to add a combination of an external additive having a relatively small particle diameter and another external additive having a relatively large particle diameter. However, the larger external additive is easily released from toner particles, and thereby the smaller external additive is embedded into the toner particles, resulting in deterioration of the fluidity of the toner. In this case, problems which occur are that image density decreases; and developing properties of the developer are changed due to the larger external additive released from the toner and remaining in the developer, resulting in occurrence of background development in which the background areas of images are soiled with toner particles.
In addition, it is well known that when an external additive released from toner particles is adhered to carrier particles, the chargeability of the carrier deteriorates depending on the properties of the external additive. In this regard, it is known that when the external additive has a low electric resistance, the charging ability of the carrier seriously deteriorates. Further, an external additive having a relatively large particle diameter and released from toner particles tends to easily form a film on the image bearing member (such as photoreceptors). In this case, blurred images are formed particularly under high temperature and high humidity conditions. In attempting to prevent an external additive having a relatively large particle diameter from being released from toner particles, several techniques have been proposed. For example, Japanese patent application publication No. 2004-246057 discloses a method in which a particulate resin having a relatively large particle diameter is mixed with toner particles for 2 minutes using a hybridization system, in which the rotor is rotated at a revolution of 12000 rpm. According to the present inventors' study, it is found that the charge properties of the toner particles to which such a particulate resin is fixed by this method deteriorate.
Because of these reasons, a need exists for a high speed image forming apparatus, which uses a non-contact AC roller charging method and which can stably produce high quality images over a long period of time without causing the filming problem and the problem in that the chargeability of the carrier deteriorates due to the released external additive.